Dust on image, using alpha node on a mask
Alpha over and Math nodes

When you want to overlay an image with transparency to another, the Alpha over node is perhaps a better coiche than any other node.

From the manual's page:


  Use this node to layer images on top of one another.




Example of usage:



Note that you don't even need to set the overlayed image's alpha, it is already taken into account.



One of the possible way to control the strenght of the mix could be lowering the incoming overlaying factor with a Math node:



While before the range of the factor was from 0 to 1, now are linearly remapped from 0 to 0.4, resulting in a softer effect.